To provide a temperature measuring device capable of measuring accurately and surely the temperature of the inside of a fine space.

This device is equipped with a container 5 constituted so that a fluid including a fluorescent material is inputted and circulated from one side of a fine passage 20 and can be recovered from the other side, a fixing stand 6 formed so that the container 5 can be moved in the two-dimensional direction, a laser light generation device 1 (light irradiation means), a photodetector 9 (photodetection means), and an operation processing device 10 (operation means). The fluid flowing in the fine passage 20 is irradiated intermittently with light from the laser light generation device 1, and the fluorescent material in the fluid is excited and allowed to generate fluorescence, and the fluorescence intensity is detected by the photodetector 9, and a damping characteristic of the fluorescence intensity is measured by the operation processing device 10, to thereby calculate the temperature of the liquid circulating in the fine passage 20.